Electron beam recording medium with amino-azo indicator and halogenated polymer coating



United States Patent Ofiice ELECTRON BEAM RECORDING MEDIUM WITHAMlNO-AZO INDICATOR AND HALOGENATED POLYMER COATING William 0. Ney, Jr.,Mahtomedi, and Joseph A. Wiese, Jr., St. Paul, Minn, assignors toMinnesota Mining and Manufacturing Company, St. Paul, Minn, acorporation of Delaware No Drawing. Filed Sept. 23, 1963, Ser. No.310,923

5 Claims. (Cl. 117-230) The present invention relates to media forrecording intelligence, particularly for the recording of intelligenceby means of an electron beam, and to the process in which such media areused.

Cathode ray tubes with phosphor screens having a visible responseessentially instantaneous with electron beam impingement have beencommonly used for transducing transmitted intelligence in electricalform into a visual form. However, the image on the phosphor screens ofsuch tubes is transistory and cannot be stored for any length oftime-without auxiliary means for providing a permanent record. Numerousefforts have been directed to recording permanently the transitoryimages on cathode ray tubes by various electronic and optionalphotographic methods. Special tube faces have been suggested to permitthe use of electrostatic development of special papers passedcontinuously over the tube face, but these tube constructions frequentlyhave produced records with poor resolution due to various forms ofdistortion associated with the system. The recording of a light image ona medium containing an acid-base type indicator dye and a halogenatedcompound which produces hydrochloric acid under the influence of light,has been described in the literature. However, applicant has found thatsuch media, when placed under vacuum conditions for direct electron beamimpingement, tend to lose volatile reactants, e.g. the relatively lowmolecular weight halogenated compounds.

It is an object of this invention to provide an electron beam recordingmedium which can be used under vacuum conditions and which can beactivated with direct electron beam impingement.

It is another object of this invention to provide an electron beamrecording media which can provide relatively stable storage ofintelligence over an extended period of time.

Still another object of this invention is to provide a recording mediumfor reliable and efiicient recording of images formed by an electronbeam.

Yet another object of this invention is to provide a process fortransducing electrical information into a visual record.

A further object of this invention is to provide means for recordingfacsimile or electrical intelligence.

Other objects and advantages will be apparent from the followingdescription.

In accordance with this invention the above and other objects arerealized by providing a recording media which comprises a substrate,preferably an electrically conductive substrate, on which issuperimposed a layer containing an oil soluble amino azo indicator dyein basic form and a highly halogenated polymeric binder. Although theindicator is generally homogeneously disposed throughout the halogenatedpolymeric binder, and is preferably dissolved therein, it may also beprovided as a localized coating on or in the top surface of thepolymeric binder.

The oil soluble amino azo indicator dyes change color at a pH of fromabout 7, preferably in the pH range of 2 to 4, and have relatively lowvolatility, e.g. a boiling point above about 90 C. at atmosphericpressure. These indicators become quinoid in the presence of acid andare characterized by their good sensitivity and, in the case of PatentedFeb. 27, 19 8 Congo red A, ability to form very stable images. The oilsoluble amino azo indicators have been found to be outstanding, sincethe presence of suflicient water solubilizing groups (e.g. acyl groupsand their salts) to adversely affect oil solubility tends to reduce theelectron beam sensitivity of the film. Solubility of the indicator inthe highly halogenated polymeric binder normally permits optimum resultswith a minimum concentration of indicator.

The highly halogenated polymers which liberate hydrogen halide uponelectron beam bombardment should be normally solid and of sufiicientlyhigh molecular weight to prevent their volatilization (i.e. above 1000preferably above 10,000 number average molecular weight), the filmforming, and contain, in addition to hydrogen, from about 25 to about 73weight percent of labile chlorine or bromine. For ease of coating theelectrically conductive substrate the polymers desirably are soluble inconventional organic solvents such as tetrahydrofuran, acetone, 2-butanone, methyl ethyl ketone, etc., although other solvent systems canbe used for the more difficultly soluble polymers such as polyvinylchloride and polyvinylidene chlorine. Solubility can be adjusted to someextent by employing copolymers, a balance being achieved between halogencontent and copolymer solubility. Vinylidene chloride copolymers withsuch monomers the aliphatic acrylates (e.g. n-butyl acrylate, methylacrylate, ethyl acrylate, hexyl acrylate, methyl methacrylate,beta-chloroethyl acrylate, etc.), acrylonitrile, vinyl choride, vinylacetate, vinyl butyrate, etc, are preferred highly halogenated polymersystems. Ethylenically unsaturated monomers with a high halogen content,such as 1,1,3,3,3-pentachloropropene-l, fluorotrichloroethylene,l,l-difluoro-2,2-dichloroethylene, trichloroethylene, etc. copolymerizedwith vinyl or vinylidene chloride or bromide or with the aliphaticacrylates can also be employed. Halogenated aromatic polymers areconsiderably less effective than the halogenated aliphatic polymers,although the copolymerization of a suitable halogenated aliphaticmonomer with an aromatic monomer (e.g. styrene, vinyl toluene, vinylcarbazole, etc.) selected for its solubility characteristics issuitable. With the preferred vinylidene chloride polymers the chlorineconcentration ranges from about 25 to about 73 percent, preferably fromabout 40 to about 70 percent by weight. With the vinyl chloride polymersthe chlorine concentration ranges from about 35 to 55 percent,preferably from about 20 to about 55 percent by weight of the polymer.Although the halogenated polymers are desirably deposited from solutionas a film on the electrically conductive substrate, they may also bedeposited from a latex or intimate dispersion. With those polymers whichtend to decompose slowly in the presence of ordinary light andatmospheric oxygen, anti-oxidants and other stabilizers may be added toimprove good storage life.

Since the highly halogenated polymer serves as a rela-' tivelynon-volatile source of hydrohalic acid, no other brominated orchlorinated compounds which liberate acid under electron beam exposureare required in the film. In fact, the monomeric or non-polymerichalogenated compounds areundesirable not only because of their highervolatility in vacuum but also because of their frequently encounteredsensitivity to moisture, air oxidation and photodecomposition undervisible light and their adverse effect on the physical properties of thefilm, such as by dilution of the polymeric vehicle. Materials which arehighly sensitive to visible light, e.g., silver halide, zinc oxide,etc., are also undesirable, since the media construction should beessentially stable to visible light. Slight fading of certain indicatorsupon extended exposure to visible light is unobjectionable.Non-polymeric sources of hydrogen halide and materials sensitive tovisible light are therefore excluded from the sheet.

The media "of this invention may be prepared by mixing a minor amount ofthe acid sensitive amino azo indicator dye with a solution of the highlyhalogenated polymer and coating the resulting admixture as a thin film,i.e., usually from 1 micron to several mils, preferably from about 2 toabout microns, onto the electrically conductive substrate. Aluminumfoil, metal coated plastic, conductive papers, etc., can be used for theelectrically conductive substrate. If a transparency is desired or if itis desired to view the image through the backing, a thin, lighttransmissive aluminum vapor coated plastic (e.g., polyethyleneterephthalate) or a conductive glass (e.g., Nesa glass) substrate may beused in conjunction with a light transmissive, electron beam sensitivecoating. Many of the highly halogenated polymers may be made even morerelatively light transmissive in the form of a thin film. For eachequivalent weight of acid sensitive amino azo indicator from about 1 toabout 1000 acid equivalents of the halogenated polymer are employed,although the ratio of these ingredients varies with the particularindicator dye, and its acid sensitivity, which is employed. Otheradditives, e.g., plasticizers, oxidizing agents, etc., may beincorporated into the electron beam sensitive coating, provided they donot serve to liberate acid under the electron beam. Additional films orcoatings may be provided on the electron beam sensitive layer to protectit from abrasion, etc., provided they are relatively transmissive to theelectron beam.

With the media described above a color change is generally observed inthe sensitive coating immediately upon electron beam impingement orshortly thereafter upon exposure to air, thereby providing a visiblerecord. In some instance the image can be erased by heating the media toabout 100 C. to 150 C. for approximately 30 seconds, the color changebeing probably due to the volatilization of the acid and an increase ineffective pH of the media. Erased media of this type can be reused forelectron beam recording, although subsequent depletion of the polymericacid source eventually reduced the efiiciency of the recording.

It is also within the scope of this invention to impinge the electronbeam on a media comprising the highly halogenated polymer film on anelectrically conductive substrate, leaving the amino azo dye out of thepolymer film. After the exposure to the electron beam, the liberatedacid in the imaged areas can be subsequently developed by contacting theexposed surface with an amino azo indicator dye. A separate developmentroller or bath may be used for this post development step, or a secondfilm construction incorporating or carrying the indicator dye can bebrought into contact with the exposed surface of the media. Oneadvantage of a post development using an indicator-containing film isthe ability to prepare multiple copies.

If the electron sensitive layer can be supported on and contiguous withan electrically conductive roll or plate during beam exposure, theelectrically conductive substrate of the recording media can beeliminated as an integral part of the media construction. However, goodelectrical contact between the electron sensitive layer and a conductivebacking, normally maintained at ground potential, is best achieved withan integral or unitary media construction including an electricallyconductive layer contiguous with the electron beam sensitive layer.

Although the foregoing description has been directed to electron beamrecording, the media described are also sensitive to other forms ofrelatively high energy irradiation which causes the liberation ofhydrohalic acid from the highly halogenated polymer, and alpha, beta andgamma radiation as well as X-rays and ion particles and ultravioletlight below 3,000 angstroms may also be em ployed at appropriate energylevels.

Since ultraviolet light below 3,000 angstroms is similar to an electronbeam in its effect on the recording media of .this invention, a simplestandard test procedure has been developed to assist in the selectionand definition of the preferred highly halogenated polymers and aminoazo indicator dyes for use in the electron beam sensitive layers of themedia. The suitability of a film forming halogenated polymer isdetermined by adding 5 milligrams of Congo red A to 1.0 milliliter of a20 weight percent solution of the polymer in a suitable solvent, such astetrahydrofuran. This solution is then knife coated onto a celluloseacetate, polyethylene terephthalate or glass backing to provide a dryfilm of 0.1 mil thickness. A sample of this dry film is placed at asuificient distance from an ultraviolet light source to provide about0.08 watt per square centimeter of radiant energy of 2,000 to 3,000angstroms wavelength. The sample is irradiated for a period from 2 to 30seconds. Generation of a blue color indicates a halogenated polymercontaining labile halogen useful in the electron beam recording media ofthis invention. The same standard test procedure is modified forselection of a particularly suitable acid sensitive amino azo indicatordye by using a 20 weight percent solution of vinylidene chlorideacrylonitrile copolymer 10 mol ratio) and 5 milligrams of the acidsensitive indicator dye, a strong color change being desired after theultraviolet exposure.

The following examples will serve to illustrate the recording media andprocesses of this invention.

Example 1 To 4 ml. of a 20% (by weight) solution of vinylidenechloride/n-butyl acrylate copolymer (90/ 10 mol ratio, approximately 60%chlorine) in tetrahydrofuran, was added 10 mg. of Congo red A dye(unsulfonated Congo red prepared from tetrazotized benzidine andl-naphthyl amine) and 4 drops of cyclohexanone. This solution was usedto prepare a film of 0.1 mil dry thickness when it was knife coated onto3 mil aluminum vapor coated polyethylene terephthalate film.

An electron beam recording made with a scanning rate of 60 fields/see,10 kv., 5 microampere beam current, 10 micron spot size and 50microsecond/scan line (262.5 lines/field) produced a direct print outblue image on an orange background. The individual scan lines weredistinctly visible under a microscope. The change in optical densitybetween background and image was about 0.5 optical density units.

Congo red A has the following structure:

NHz NHz Example 2 To 4 ml. of a 20% (by weight) tetrahydrofuran solutionof a polymer of vinylidene chloride/n-butyl acrylate (92/8 mole ratio,approximately 62% chlorine) was added 20 mg. of Congo red A dye and 4drops of cyclohexanone. This solution was used to prepare a film of 0.1mil dry thickness by knife coating onto 3 mil aluminum vapor coatedpolyethylene terephthalate film. When it was recorded as above a similarimage was obtained having an optical density diiference of 0.6 betweenbackground and image.

Example 3 Films prepared and recorded as above using 20% tetrahydrofuransolutions of vinyl chloride/vinyl acetate copolymer (43% by weightchlorine); vinylidene chloride/ acrylonitrile copolymer (54% chlorine);vinylidene chloride/acrylonitri-le copolymer (67% chlorine); tosylatedpolyvinyl alcohol; chlorinated polypropylene; copolymers of vinylidenechloride With any of methyl acrylate, 8- chloroethyl acrylate, vinylchloride, vinyl acetate or vinyl butyrate, into each of which 1-20 mg.of Congo red A dye were added (per 200 mg. polymer) yielded direct printout images. The optical density difference between image and backgroundvaried depending on the specific polymer and the amount of dye used.However, the films all gave differences lying in the range of 0.1-0.3optical density units.

Example 4 To 4 ml. of a (by weight) solution of a polymer of vinylidenechloride/n-butyl acrylate (90:10 mole ratio; approximately 60% chlorine)in tetrahydrofuran was added 20 mg. of 4-phenylazodiphenyl amine and 4drops of cyclohexane. This solution was used to prepare a film of 0.1mil dry thickness when it was knife coated onto 3 mil aluminum vaporcoated Mylar film.

An electron beam recording made with a scanning rate of 60 fields/see,10 kv., 5 microampere beam current, 10 micron beam spot and 50microsecond/ scan line showed a direct print out, brown image on a paleyellow background. The individual scan lines were distinctly visibleunder a microscope. The change in optical density between image andbackground was about 0.5 optical density units.

' Example 5 Using a vinylidene chloride/n-butyl acrylate copolymer(about 60% chlorine) and the same procedure for coating and recording asoutlined above in Example 1 the following amino azo dyes were tested:

phenyl mercuric acetate.

Example 6 A recording medium having outstanding characteristics wasprepared by adding 0.030 g. of Congo red A to 3.5 ml. of a solution of5.00g. of a copolymer of 87% vinyl chloride and 13% vinyl acetatedissolved and diluted to 100 ml. with methylethyl ketone. To thissolution was added 1.5 ml. of methylethyl ketone, 4.0 ml. of n-amylacetate and 1.0 ml. of 1,1,2,2-tetrachloroethane (these last twosolvents are added to decrease the evaporation rate and to obtainunblushed coatings). The resulting clear red solution was then dipcoated onto aluminum foil at a rate of 43 ft. per second. The coatedfoil was pulled vertically into a 1 /2 inch diameter tube where it wasallowed to dry. Solution 1.25 to 5.00 g. per 100 ml. solvent gave drycoating thickness of 3 to 5 microns. The dried, coated foil was thenevaluated for sensitivity using an electron gun having an unmodulated,non-sweep beam from about 2,000 to 15,000 volts and beam currents up toabout microamperes. The spot size was about 0.50 mm. A sensitivity ofabout 0.1 microampere-second/rnm. was obtained. This corresponds to arecording sensitivity of about 4 megacycles per second.

Media using other concentrations of polymer and Congo red A wereprepared and successfully tested for electron beam sensitivity.

The n-arnyl acetate and tetrachloroethane added to eliminate blushing ofcoatings may be replaced by other materials such as cyclohexane ordimethyl Cellosolve.

In general, those acid sensitive amino azo indicators with a markingenergy of below 10 microampere-seconds/ mm preferably below 0.3microampere-second/mm are superior for the recording sheets of thisinvention.

Example 7 An admixture of a halogenated copolymer (87 mol percent vinylchloride and 13 mol percent vinyl acetate) and methyl yellow (1 part dyeto 10 parts by weight of copolymer) was coated onto a metal substrate toprovide ride and 12 mol percent vinyl acetate, the weight ratio ofindicator to hinder being about 1 to 5.

TABLE I.-AMINO AZO DYES IN ELECTRON SENSITIVE MEDIA Energy to MarkExample Indicator (microamperes, Color Response sec, at 2,000 volts) CHA O-N: -N 0. 2 Red on yellow.

B C N=NC NH2 03 Do.

- i CH3 0 s: N 2 (i1 0 OH (Methyl Red) H2 NHz D N=N 10 Blue on red, poorsolublllty.

' THF S 03H S 0 H NH 7 N H2 E 2 Blue, on red excellent stability.

1 Red on yellow.

Energy to Mark (mieroamperes, Color Response see, at 2,000 volts)Indicator Example Blue on red.

Red to yellow.

2 Orange on yellow.

Tan on yellow.

NHC O OH;

Red on yellow.

M CH

Purple on yellow.

N CH

Red to yellow.

O.. HOOC- 03 Purple on orange.

NHa

P OHQO Blue on purple.

. 03 Blue on red.

Brown on tan.

Orange on yellow.

Purple on yellow.

Noa

Brown on tan.

OCH

Pale yellow on tan.

Purple on yellow,

excellent stability,

Example Indicator Various other embodiments of the present inventionwill be apparent to those skilled in the art without departing from thescope thereof.

The following is claimed:

1. A recording medium which comprises an electrically conductivemetallic substrate and, superimposed thereon, a layer which isessentially insensitive to visible light, which is stable under vacuumconditions used in electron beam recording and which comprises anacid-sensitive oil soluble amino azo indicator in basic form and capableof changing color at a pH below about 7 and, as the sole remainingconstituent capable of liberating hydrogen halide under exposure to anelectron beam, a normally solid, highly halogenated polymer having amolecular weight of at least about 1000 and having at least 25 weightpercent of labile halogen selected from the group consisting of chlorineand bromine.

2. The recording medium of claim 1 in which said film has from about 1to about 1000 acid equivalents of said halogenated polymer perequivalent of acid sensitive indicator.

3. The recording medium of claim 1 in which said acid sensitiveindicator is Congo red A.

Energy to Mark (mloroamperes,

Color Response see, at 2,000 volts) 2 Red on tan.

.2 Yellow on tan.

4. The recording medium of claim 1 in which said highly halogenatedpolymer is a copolymer of vinylidene chloride and an aliphatic acrylate.

5. The recording medium of claim 1 in which said highly halogenatedpolymer is a copolymer of vinyl chloride and vinyl acetate.

References Cited UNITED STATES PATENTS WILLIAM D. MARTIN, PrimaryExaminer.

E. J. CABIC, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,370,981 February 27, 1968 William O. Ney, Jr., et al.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 2, line 13, "the film" should read be film Columns 5, 6, 7, 8, 9and 10, TABLE I, in the sub-heading, line 2 thereof, "microamperes,"each occurrence, should read microampe res Columns 7 and 8 same Table Iopposite Example "R", the indicator formula should appear as shownbelow:

Same Table I opposite Example "X", the formula should appear as shownbelow:

OCH

OCH

Signed and sealed this 11th day of November 1969.

(SEAL) Attest:

EIZIZARgMTLETCHERJR. WILLIAM E. SCHUYLER, JR.

es ing Officer Commissioner of Patents

1. A RECORDING MEDIUM WHICH COMPRISES AN ELECTRICALLY CONDUCTIVEMETALLIC SUBSTRATE AND, SUPERIMPOSED THEREON, A LAYER WHICH ISESSENTIALLY INSENSTIVE TO VISIBLE LIGHT, WHICH IS STABLE UNDER VACUUMCONDITIONS USED IN ELECTRON BEAM RECORDING AND WHICH COMPRISES ANACID-SENSITIVE OIL SOLUBLE AMINO AZO INDICATOR IN BASIC FORM AND CAPABLEOF CHANGING COLOR AT A PH BELOW ABOUT 7 AND, AS THE SOLE REMAININGCONSITUENT CAPABLE OFLIBERATING HYDROGEN HALIDE UNDER EXPOSURE TO ANELECTRON BEAM A NORMALLY SOLID, HIGHLY HALOGENATED POLYMER HAVING AMOLECULAR WEIGHT OF AT LEAST ABOUT 1000 AND HAVING AT LEAST 25 WEIGHTPERCENT OF LABILE HALOGEN SELECTED FROM THE GROUP CONSISTING OF CHLORINEAND BROMINE.